When it's cold, we do get some humid days (not so much on sunny cold days), but the A7 is also a dehumidifier.
We have metal items stored in the attic, and there doesn't seem to be much of a rust problem up there.
I don't think that pumping attic air to the basement is going to add overwhelming humidity.
The benefits of drying the basement air is one of the things I'm looking forward to.
My machine shop tools are rust magnets..
The fancy new Zenith dehumidifier we got last year is going to find a new home at my daughter's house.